Crystal symmetry and superlattice reflections in spin-Peierls system TiOBr

We have demonstrated the direct observation of the crystal symmetry by electron and synchrotron X-ray diffraction analyses. Although the crystal structure at room temperature can be explained by orthorhombic Pmmn symmetry in our powder X-ray diffraction analysis, we detected weak reflections which should be the forbidden reflections of the Pmmn symmetry by electron and synchrotron X-ray diffraction analyses using single-crystalline TiOBr samples. We concluded that the crystal symmetry of TiOBr is different from the orthorhombic Pmmn symmetry. In this report, we discuss the crystal symmetry on the basis of the subgroup of orthorhombic Pmmn at room temperature and low temperature below T_c1(=27 K).